The present invention is directed to a cement admixture composition capable of causing the combined effects of inhibiting drying shrinkage of concrete compositions while enhancing the compressive strength of the fully set composition. The present invention further provides an improved concrete composition structural product.
Specifically, the present invention is directed to a cement admixture composed of a synergistic combination of certain alkylene glycols and fumed silica as fully described hereinbelow.
Hydraulic cement compositions, such as mortar (cement, small particulate, e.g. sand, and water), or concrete (cement, small particulate, large particulate, e.g. gravel, and water), have certain properties which substantially effect their durability. These properties include shrinkage which normally occurs during drying of the cement composition. In addition, mortars and, in particular, concrete composition are used for structural applications where enhanced strength of the cured structural member is highly desired.
Conventional hydraulic cement compositions display a decrease in volume with setting and drying of the cast composition. Although the magnitude of the volume decrease is normally small, it is of extreme importance. This shrinkage results in cracks and other defects which lower the serviceability and durability of the resultant structure. The cracks provide a path for air to penetrate into the structure, promoting carbonation of the cement and corrosion of the metal reinforcing bars contained therein. Further, the cracks provide a means for water to seep into and through the structure. Such water entry further deteriorates the structure through freeze-thaw cycling pressures exerted on the cement structure over its life. It is highly desired to provide a cement which exhibits high strength and is not subject to deterioration effects due to shrinkage and freeze-thaw cycling.
Various attempts have been made to avoid the cracking phenomenon caused by drying shrinkage. These include providing joints in the cement structure to concentrate the site of crack formation at the joint and, thereby, minimize such formation at other portions of the structure. Such joints are expensive to install; are not applicable to certain structures such as vertical walls, pillars and the like; and merely concentrate the area of cracking but do not alleviate it.
Other attempts include varying the composition of the cement, varying the methods of manufacture of concrete mix and varying the ballast material used in forming the resultant concrete structure. None of these attempts have resulted in a satisfactory solution. For example, cements have been formulated with expansive admixtures in attempts to counter the shrinkage of the concrete. However, it is difficult to determine the proper amount of expansive admixture required to counter the drying shrinkage which develops. The use of such materials thereby give rise to unpredictable results.
With respect to overcoming the drying shrinkage of cement compositions, such as concrete compositions, the literature teaches that various oxyalkylene adducts are suitable for this purpose. For example, U.S. Pat. Nos. 3,663,251 and 4,547,223 suggest the use of compounds of the general formula RO(AO).sub.n H in which R may be a C.sub.1-7 alkyl or C.sub.5-6 cycloalkyl radical, A may be C.sub.2-3 alkylene radicals and n is 1-10, as shrinkage reducing additives for cement. Similarly, U.S. Pat. No. 5,174,820 suggests terminally alkyletherified or alkylesterified oxyalkylene polymers as useful for shrinkage reduction. Still further, Japanese Patent Application 58-60293 provides the suggestion that shrinkage reduction of cement can be accomplished by the addition thereto of compounds which are aliphatic, alicyclic or aromatic group terminated oxyethylene and/or oxypropylene repeating chain compounds.
One of the main advantages of using concrete compositions, to form architectural structural members is their ability to be cast into a desired form which is capable of exhibiting high compressive strength. With this in mind, the artisan does not desire to utilize admixtures or other ingredients which cause a decrease in such strength.
It is highly desired to provide a cement admixture which is capable of inhibiting drying shrinkage of structural concrete compositions while enhancing the compressive strength of the resultant cured structure.
Alkylene glycols and glycerols have been used in combination with cement compositions for particular purposes. For example, these materials have been added to inhibit water crystal formation when casting in cold climate conditions or to inhibit the rate of evaporation of water in cement slurries used in high temperature well bore hole applications. Further, these additives have been used to provide a layer above cast, unset cement composition to inhibit evaporation of water at the surface portion of the structure and thereby enhancing the hydration of the cement at that portion of the formation.
The above compounds, when made part of an unset composition, cause the resultant cured composition to exhibit lower compressive strength than its untreated counterpart. When the cement composition is a concrete which is used to provide architectural structural members for buildings, parking garages, bridge decks and the like, it is essential that the concrete member exhibit high compressive strength than its untreated counterpart. Therefore, shrinkage reducing additives which decrease the strength of the cured product have not found favor even though they inhibit cracking in the member as discussed above.